The present invention relates to baluns, and more particularly, to a tapered double balun that may be used with four-terminal antennas that are fed using spiral-mode number 2.
A balun is a device that converts a balanced transmission line to an unbalanced transmission line. For broad bandwidth, tapered baluns have been used for many years. Furthermore, currently available technologies for producing circular polarized omnidirectional antenna patterns require a complex stripline circuit that is expensive, gain-reducing, and bulky. Currently available technologies cannot easily produce an antenna pattern like that of a whip antenna from a very low-profile antenna.
Tapered baluns are discussed by Duncan, J. W., and Minerva, V. P., “100:1 Bandwidth balun transformer,” Proc. IRE. Vol. 48, No. 2, 1960, pp. 156-164. The tapered balun is an excellent feed for very broadband, balanced antennas, such as spirals and sinuous antennas. Its bandwidth is limited only at the low-frequency end, which can be made arbitrarily low by making the taper longer. Furthermore, the tapered balun can be easily designed and fabricated in an inexpensive printed-circuit medium.
A four-arm spiral antenna that operates only in Mode 2 (see Corzine, R. G., and J. A. Mosko, Four-Arm Spiral Antennas, Artech House, 1990) has only two independent electrical terminals, since opposite arms theoretically can be connected together. However, practically, it is not obvious how to achieve this connection in a rugged and inexpensive way. A tapered coplanar-line feed has been reported by Gschwendtner, E., et al, in “Spiral antenna with frequency-independent coplanar feed for mobile communication systems” APS International Symposium, 1999, IEEE, Vol. 1, 11-16 Jul. 1999, p. 560-563, but a cavity will interfere with it, and the authors do not show that it is balanced at the feed point. It would be desirable to have a balun that solves the above-discussed problems and provides an improved technique to excite Mode 2 in four-arm spiral antennas.